Why do christians believe jesus was so important?

In the resurrection, the whole focus on Jesus is changed. He is understood to be a divine incarnation, the title Son of Man and the Kingdom of God all change. Jesus explained all of this on the way to Emmaus.

The teachings of Jesus on how to live, how to have right relationship with God, how to love thy neighbour, all take on a new meaning, a new importance, for this is how the Kingdom of God is made present, in this day, this age.

The Resurrection is the promise of freedom from the Sin of Adam, and opens the Gates of Heaven to all who believe and all who live a good life with the Golden Rule.